1. Field of the Invention
The present invention relates to a digital camera and a power supply apparatus suitable therefor.
2. Description of the Related Art
Recently, a digital camera has been used often in place of a conventional camera utilizing a film. Additionally, in the digital camera, it has been aimed to allow a lens-barrier motor, a zoom motor and a focus motor and the like to complete their predetermined operation as soon as possible at the time of activation or photographing to reduce the time lag and thereby to enhance user convenience. As one of the ways to accomplish the aim, for example, motor driving power is increased to drive the motor at high speed, or a plurality of motors is driven simultaneously, to achieve reduction of operating time of the camera. As a result, load power at the time of operation of the motor is increased and hence a large load is applied to a power supply.
As conventional technique to reduce such load in the power supply, JP-A 2001-208956 discloses a digital camera capable of adjusting the time required for starting-up thereof corresponding to capacity of a power supply currently used. According to this conventional technique, it discloses the technique in which the capacity and kind of the power supply are judged on the basis of a voltage value of the power supply inputted through a bus to control the driving of a DC motor as a motor for zooming and a stepping motor as a motor for focus in accordance with the capacity and the kind of the power supply judged.
However, as well as in the conventional technique disclosed in the above mentioned JP-A 2001-208956, there are many portions in the digital camera that require large load power such as a DSP (Digital Signal Processor) and a memory, other than the load power for the motor. Accordingly, in the digital camera in which same power source is used with respect to those portions, there is a possibility that power supply voltage drops by increase in the load power of the motor and thereby operation of the digital camera itself cannot be carried out. Even if the digital camera is barely operated in such case, the problem occurs that the life of battery is shortened in a case where a battery is used as the power supply.
Also, as disclosed in Japanese Patent No. 2794631, measures have been taken in which, when driving the plurality of motors simultaneously, each load current is reduced so as not to increase the load power. However, this makes a period of operating the motor long.
Meanwhile, there are three reasons for generation of a voltage variation in the power supply due to load fluctuation, first, for lack of supply capacity of the power supply, second, for lack of conversion capacity of a power conversion circuit in the power supply, and third, for too much loads. Among these reasons, it can be said that the lack of capacity of the power conversion circuit is the major technological issue with respect to the power supply.
The power conversion circuit constantly monitors output voltage, and operates in such a manner as to increases supply of power when the load is increased and decreases the supply of power when the load is decreased. However, in the rapid change of the load, for example if the load is increased drastically, the output voltage falls since the power cannot be supplied sufficiently, whereas if the load is decreased drastically, the output voltage rises due to excessive supplying of power. There are methods to change oscillation cycle and change duty of ON/OFF oscillation, in order to correspond to the drastic load fluctuation. However, there is a limitation in terms of software and hardware to allow the oscillation cycle to be faster, and 100% is the limitation to make “ON-duty” to be longer in the changing of the duty, thus the fall of the output voltage cannot be suppressed at some point. Therefore, it can be said that the operational limitation in terms of the software and the hardware is the limitation of the capacity in the power conversion circuit.
As conventional technique for the power supply, JP-A H6-119074 discloses the technique for reducing a variation in output voltage and a variation in input voltage caused by the load fluctuation. According to this conventional technique, a conventional power supply apparatus is combined with a flow diverter and an alternating current detecting circuit to construct a power supply apparatus. The variation in output current flowing through a load is detected by the alternating current detecting circuit, while the alternating current detecting circuit outputs an output signal which complies with the load fluctuation to the flow diverter. The flow diverter changes amplitude of the output signal in accordance with the output signal from the alternating current detecting circuit to divert current to be supplied from the power supply apparatus to the load.
In order to correspond to such voltage variation, the conventional power supply apparatus provided with the power conversion circuit uses a large capacity of condenser for the output or accelerates a response of feedback of the output as a way to suppress the voltage variation caused by the load fluctuation. However, use of the large capacity of condenser causes various adverse effects to generate such as increase of rush current, delay in rising and falling, rise in cost and increase in mounting spaces. In addition, there is a limitation to make the response of the feedback of the output faster, and even if the acceleration of the response in the feedback of the output is obtained, there is a problem that power consumption increases significantly because the response is accelerated.
Also, since the conventional technique disclosed in the above mentioned JP-A H6-119074 is the technique which carries out the feedback by regarding that the degree which the load fluctuates is the alternating current-like variation, and there is the limitation in order to accelerate the response in the feedback of the output, it also has the problem similar to the above described conventional technique.